Metabolic regulation of the trehalose content of vegetative yeast

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Abstract
We have investigated the mechanism by which heat shock conditions lead to a reversible accumulation of trehalose in growing yeast. When cells of S. cerevisiae MI growing exponentially at 30°C were shifted to 45°C for 20 min, or to 39°C for 40 min, the concentration of trehalose increased by about 25-fold; an effect reversed upon lowering the temperature to 30°C. This was compared to the more than 50-fold rise in trehalose levels obtained upon transition from the exponential to the stationary growth phase. Whereas the latter was paralleled by a 12-fold increase in the activity of trehalose-6-phosphate synthase, no significant change in the activities of trehalose-synthesizing and -degrading enzymes was measured under heat shock conditions. Accordingly, cycloheximide did not prevent the heat-induced accumulation of trehalose. However, the concentrations of the substrates for trehalose-6-phosphate synthase, i.e. glucose-6-phosphate and UDP-glucose, were found to rise during heat shock by about 5–10-fold. Since the elevated levels of both sugars are still well below the K m-values determined for trehalose-6-phosphate synthase in vitro, they are likely to contribute to the increase in trehalose under heat shock conditions. A similar increase in the steady-state levels was obtained for other intermediates of the glycolytic pathway between glucose and triosephosphate, including ATP. This suggests that temperature-dependent changes in the kinetic parameters of glycolytic enzymes vary in steady-state levels of intermediates of sugar metabolism, including an increase of those that are required for trehalose synthesis. Trehalose, glucose-6-phosphate, UDP-glucose, and ATP, were all found to increase during the 40 min heat treatment at 39°C. Since this also occurs in a mutant lacking the heat shock-induced protein HSP104 (Δhsp104), this protein cannot be involved in the accumulation of trehalose under these heat shock conditions. However, mutant Δhsp104, in contrast to the parental wild-type, was sensitive towards a 20 min incubation at 50°C. Since this mutant also accumulated normal levels of trehalose, we conclude that HSP104 function, and not the accumulation of trehalose, protects S. cerevisiae from the damage caused by a 50°C treatment.